In one of the commercially used methods for manufacturing titanium dioxide pigment particles, known as the chloride process, titanium tetrachloride (TiCl4) is reacted with an oxidizing gas, such as oxygen, air, etc., and with certain additives in a plug flow reactor to form titanium dioxide and chlorine gas:TiCl4+O2→TiO2+2Cl2 
The TiO2 particles are subsequently separated from the chlorine gas. Known additives are AlCl3 as a rutilizing agent and steam or alkali salts as a nucleating agent.
Owing to the high activation energy of TiCl4 oxidation, the educts must be heated to such a degree that an adiabatic mixed temperature of the educts of approx. 800° C. is reached before the onset of the reaction so that the reaction takes place completely. The oxidation reaction is highly exothermal, meaning that, following complete, adiabatic conversion, the temperature of the product stream is roughly 900° C. higher than that of the educts. Before the TiO2 particles are separated from the gaseous reaction products with the help of a filter, this mixture has to be substantially cooled in a cooling section in order to avoid damage to the filter.
According to U.S. Pat. No. 3,615,202, oxygen and TiCl4 are preheated and subsequently caused to react in a plug flow reactor. Since the full quantities of the educts are in each case added at one point of the plug flow reactor, this can be referred to as a single-stage method. Similarly, EP 0 427 878 B1 discloses a single-stage method in which oxygen is heated to roughly 1,500° C. to 1,650° C. and mixed with TiCl4/AlCl3 vapour having a temperature of roughly 450° C. The process is energetically unsatisfactory in both cases because, referred to the 100% energy input for heating the educts, roughly 160% thermal reaction energy is subsequently released. Roughly 210% thermal energy then has to be dissipated from the hot product stream in the cooling section. The substantial reaction enthalpy is thus not used for activating the reaction, but transferred to the cooling-water system.
In the single-stage method according to GB 969,618 TiCl4 is introduced co-axially and the hot oxygen containing gas is introduced tangentially into the reactor. The TiCl4 is introduced in gaseous or liquid form.
For the purpose of energetic optimization, EP 0 583 063 B1 describes two-stage introduction of TiCl4 into the reactor. TiCl4 with a temperature of at least 450° C. and mixed with AlCl3 is fed into the hot oxygen stream at a first inlet point, and TiCl4 with a temperature of 350° C. to 400° C. and without AlCl3 at a further inlet point.
The method according to EP 0 852 568 B1 provides for not only the TiCl4 to be added in two stages, but also the oxygen. However, the object of this method is effective control of the mean TiO2 particle size, and thus of the tone of the TiO2 pigment base material. In this case, TiCl4 vapour having a temperature of roughly 400° C. is first fed into an oxygen stream with a temperature of roughly 950° C. The TiO2 particles are formed, and particle growth takes place, in the downstream reaction zone. TiCl4 vapour heated to a lesser extent (approx. 180° C.) is added at a second inlet point. Oxygen having a temperature between 25° C. and 1,040° C. is introduced at the second inlet point, the temperature of the mixture being sufficient to initiate the reaction.
The multi-stage method according to U.S. Pat. No. 6,387,347 is additionally said to reduce agglomeration. To this end, the previously heated TiCl4 stream is split into two part streams before addition to the reactor. One part stream is oxidized in the first stage of the reactor. The second part stream is cooled by injection of liquid TiCl4 (de-superheating) and then added to the reactor. De-superheating takes place outside the reactor, the temperature not falling below the condensation temperature of the overall stream.
US 2007/0172414 A1 discloses a multi-stage method in which gaseous TiCl4 is fed into the reactor in the first stage, and liquid TiCl4 in the second stage. This method permits energy savings and improvement of the particle size range.